Control circuit of a driving circuit for regulating the switching frequency of a discharge lamp

Abstract

A control circuit controls a driving circuit of a discharge lamp. The driving circuit comprises a half bridge and a clock generator that determines the switching frequency of the half bridge. The control circuit comprises a regulator that regulates the value of the switching frequency when the value of the voltage across the lamp exceeds a threshold value.

1. A control circuit, comprising:
 a sensor structured to sense a voltage of a discharge lamp driven by a driving circuit that includes a half bridge and a circuit that determines a switching frequency of the half bridge; and 
 regulating means for regulating the switching frequency when the voltage across the lamp exceeds a threshold value, the threshold value comprising a first value and a second value, the threshold value being the first value when the driving circuit ignites and being the second value when the driving circuit runs the discharge lamp. 
 
     
     
       2. The control circuit according to  claim 1 , wherein said regulating means are structured to clamp or to increase the switching frequency during an ignition phase of said discharge lamp. 
     
     
       3. The control circuit according to  claim 1 , wherein said regulating means are structured to increase the switching frequency during a run phase of said discharge lamp. 
     
     
       4. The control circuit according to  claim 1 , further comprising means for setting a maximum time interval in which the value of the switching frequency is regulated. 
     
     
       5. The control circuit according to  claim 4 , wherein said regulating means are structured to clamp or to increase the switching frequency during an ignition phase and to turn off the driving circuit if at the end of said maximum time interval the lamp is not ignited. 
     
     
       6. The control circuit according to  claim 4 , wherein said regulating means are structured to increase the switching frequency during a run phase and to turn off the driving circuit if at the end of said maximum time interval the voltage across the discharge lamp is lower than the threshold value. 
     
     
       7. The control circuit according to  claim 1 , the first value being higher than the second value. 
     
     
       8. The control circuit according to  claim 7 , wherein said regulating means are structured to turn off the driving circuitry when the voltage across the lamp equals or overcomes said first value. 
     
     
       9. The control circuit according to  claim 1 , wherein said regulating means is structured to increase a current generated by the circuit that determines the switching frequency of the half bridge. 
     
     
       10. The control circuit according to  claim 9 , wherein said current depends on a voltage across a capacitor, said regulating means comprising further means for exponentially changing said voltage across the capacitor. 
     
     
       11. The control circuit according to  claim 10 , wherein said further means comprise a transistor having conduction terminals coupled with terminals of the capacitor and a control terminal coupled with a terminal of another capacitor, said regulating means comprising means for charging or discharge said another capacitor. 
     
     
       12. The control circuit according to  claim 11 , wherein said charging or discharging means comprise first and second current generators and first and second switches arranged between the control terminal of said transistor and the respective first and second current generators, said regulating means comprise a logic circuit structured to control said first and second switches based on the voltage across the lamp exceeding said threshold value. 
     
     
       13. A driving circuit for a discharge lamp, comprising:
 a half bridge structured to drive the discharge lamp; 
 a clock generator structured to determine a switching frequency of the half bridge; and 
 a control circuit structure to control the half bridge, the control circuit including: 
 a sensor structured to sense a voltage across the lamp; and 
 a regulator structured to regulate the switching frequency when the voltage across the lamp exceeds a threshold value, the threshold value comprising a first value and a second value, the threshold value being the first value during an ignition phase and being the second value during a run phase of the discharge lamp. 
 
     
     
       14. The driving circuit according to  claim 13 , wherein said regulator includes a clamping circuit structured to clamp the switching frequency during an ignition phase of said discharge lamp. 
     
     
       15. The driving circuit according to  claim 13 , wherein said regulator is structured to turn off the half bridge if the lamp is not ignited within a maximum time interval. 
     
     
       16. The driving circuit according to  claim 13 , the first value being higher than the second value. 
     
     
       17. The driving circuit according to  claim 13 , further comprising first and second capacitors, wherein said regulator includes:
 a transistor having conduction terminals coupled with terminals of the first capacitor and a control terminal coupled with a terminal of the second capacitor; 
 a first switch structured to charge said second capacitor; and 
 a second switch structured to discharge said second capacitor. 
 
     
     
       18. The driving circuit according to  claim 17 , wherein said regulator includes a logic circuit structured to control said first and second switches based on the voltage across the lamp exceeding said threshold value. 
     
     
       19. A method for controlling a discharge lamp, comprising:
 sensing a voltage across the discharge lamp; and 
 regulating a switching frequency of a half bridge that drives the lamp, the regulating step occurring while sensing that the voltage across the lamp exceeds a threshold value, the threshold value comprising a first value and a second value, the threshold value being the first value during an ignition phase and being the second value during a run phase of the discharge lamp. 
 
     
     
       20. The method according to  claim 19 , comprising, during a phase in which the driving circuit ignites the lamp, clamping the switching frequency for a maximum time period when the voltage across the lamp exceeds said threshold value. 
     
     
       21. The method according to  claim 20 , comprising turning off the half bridge if at the end of said maximum time period the lamp is not ignited. 
     
     
       22. The method according to  claim 20 , comprising turning off the half bridge if at the end of said maximum time period the voltage across the discharge lamp is lower than the threshold value. 
     
     
       23. The method according to  claim 19 , comprising, during a phase in which the lamp is running, increasing the switching frequency for a time period when the voltage across the lamp exceeds said threshold value. 
     
     
       24. The method according to  claim 23 , the first value being higher than the second value. 
     
     
       25. The method according to  claim 24 , comprising, during the run phase, turning off the half bridge when the voltage across the lamp equals or overcomes said first value. 
     
     
       26. A lighting device, comprising:
 a discharge lamp; 
 a half bridge structured to drive the discharge lamp; 
 a clock generator structured to determine a switching frequency of the half bridge; and 
 a control circuit structure to control the half bridge, the control circuit including: 
 a sensor structured to sense a voltage across the lamp; and 
 a regulator structured to regulate the switching frequency when the voltage across the lamp exceeds a threshold value, the threshold value comprising a first value and a second value, the threshold value being the first value during an ignition phase and being the second value during a run phase of the discharge lamp. 
 
     
     
       27. The lighting device according to  claim 26 , wherein said regulator includes a clamping circuit structured to clamp the switching frequency during an ignition phase of said discharge lamp. 
     
     
       28. The lighting device according to  claim 26 , wherein said regulator is structured to turn off the half bridge if the lamp is not ignited within a maximum time interval. 
     
     
       29. The lighting device according to  claim 26 , the first value being higher than the second value. 
     
     
       30. The lighting device according to  claim 26 , further comprising first and second capacitors, wherein said regulator includes:
 a transistor having conduction terminals coupled with terminals of the first capacitor and a control terminal coupled with a terminal of the second capacitor; 
 a first switch structured to charge said second capacitor; and 
 a second switch structured to discharge said second capacitor. 
 
     
     
       31. The lighting device according to  claim 30 , wherein said regulator includes a logic circuit structured to control said first and second switches based on the voltage across the lamp exceeding said threshold value.